In Figure 19, the main panel shows the variation in depth of drawdown within the zone of potential hydrological change for the 50th percentile (median), while the two smaller panels show extents for the 5th and 95th percentiles to illustrate the variability in model predictions due to parameter uncertainty. Table 8 summarises the areas where the additional drawdown is greater than 0.2 m, greater than 2 m and greater than 5 m for the 5th, 50th and 95th percentiles. For additional drawdown greater than 0.2 m, the area associated with the 5th percentile (528 km2) can be interpreted as representing the extent of drawdown when the model parameters reflect lower pumping rates and/or lower hydraulic conductivities, whereas the area of drawdown associated with the 95th percentile (2441 km2) also includes the predictions based on higher pumping rates and relatively conductive geological layers. This is a general guide only as the influences of the different parameters can be complex and produce a range of drawdown responses. Groundwater drawdown predictions indicate that drawdowns of greater than 5 m are very likely (greater than 95% chance; 5th percentile) due to the additional coal resource developments at Bylong, Mandalong, Ulan and Mount Arthur (Figure 19, top left). Drawdowns exceeding 5 m have at least a 50% chance of occurring at Wallarah 2, Drayton South and Moolarben (Figure 19, main panel).

The spatial distribution of drawdown under the baseline is shown in Figure 20, providing a visual comparison to the potential groundwater drawdown due to additional coal resource development in Figure 19. Under the baseline, the area with at least a 5% chance of drawdown greater than 0.2 m is 4307 km2. The area of overlap with the groundwater zone is 1619 km2 and represents the area where drawdowns due to baseline and additional coal resource developments potentially accumulate. Another 260 km2 overlaps with the surface water zone and defines the area where lagged groundwater drawdown responses from baseline developments could coincide with more instantaneous changes in streamflow due to additional coal resource development. Table 9 summarises the drawdown information in terms of area (km2) in the zone of potential hydrological change for each drawdown class in each reporting area.

Table 8 Area (km2) potentially exposed to varying levels of additional drawdown in the zone of potential hydrological change

Reporting area

Area in zone of potential hydrological change

(km2)

Area with additional drawdown ≥0.2 m

(km2)

Area with additional drawdown ≥2 m

(km2)

Area with additional drawdown ≥5 m

(km2)

5th

50th

95th

5th

50th

95th

5th

50th

95th

Upper Goulburn

511

80

146

363

35

77

162

12

48

94

Lower Goulburn

494

51

155

343

10

54

129

3

19

64

Central Hunter

1015

157

393

790

24

95

258

10

37

131

Lower Hunter

333

33

82

165

0

5

25

0

0

7

Macquarie-Tuggerah

860

207

453

780

52

230

353

10

123

228

Total

3213

528

1229

2441

121

461

927

35

227

524

The area potentially exposed to ≥0.2, ≥2 and ≥5 m additional drawdown for the 5th, 50th and 95th percentile estimates of the maximum difference in drawdown (dmax) between the coal resource development pathway (CRDP) and baseline, due to additional coal resource development. Drawdowns in the mine pit exclusion zones cannot be quantified with confidence. Some totals do not add up due to rounding.

Table 9 Area (km2) potentially exposed to varying levels of baseline drawdown in the zone of potential hydrological change

Reporting area

Area in zone of potential hydrological change

(km2)

Area with baseline drawdown ≥0.2 m

(km2)

Area with baseline drawdown ≥2 m

(km2)

Area with baseline drawdown ≥5 m

(km2)

5th

50th

95th

5th

50th

95th

5th

50th

95th

Upper Goulburn

511

211

310

394

89

208

274

35

143

235

Lower Goulburn

494

0

0

0

0

0

0

0

0

0

Central Hunter

1015

321

597

848

72

237

508

10

88

332

Lower Hunter

333

127

192

255

23

88

178

4

30

131

Macquarie-Tuggerah

860

174

225

381

84

135

178

24

83

125

Total

3213

833

1324

1879

268

668

1138

73

344

823

The extent potentially exposed to ≥0.2, ≥2 and ≥5 m baseline drawdown is shown for the 5th, 50th and 95th percentiles. Baseline drawdown is the maximum difference in drawdown (dmax) under the baseline relative to no coal resource development. Drawdowns in the mine pit exclusion zones cannot be quantified with confidence. Some totals do not add up due to rounding.

Figure 21 summarises the modelled drawdowns under the baseline and due to the additional coal resource development as log-transformed cumulative exceedance plots by area for the 5th, 50th and 95th percentile drawdown distributions. The mine pit exclusion areas are not included. It can be seen that a drawdown of at least 2 m due to the additional coal resource development (right panel) is very likely to occur over about 80 km2, but very unlikely to occur over an area exceeding 700 km2 (as per Table 8). Because the data are not classified, details within the classes can be discerned: drawdowns of at least 1 m due to baseline development are very likely over about 300 km2 and due to additional coal resource development over 150 km2; drawdowns greater than 50 m are very unlikely due to additional coal resource development, but there is a 5% chance of drawdowns between 50 and 100 m over 50 km2 due to baseline development.

Baseline drawdown is the maximum difference in drawdown (dmax) under the baseline relative to no coal resource development. Additional drawdown is the maximum difference in drawdown (dmax) between the coal resource development pathway (CRDP) and baseline, due to additional coal resource development.

When local-scale hydrogeological information is used to constrain model results, the range of predicted drawdown extents can reduce substantially from that predicted using the full set of simulations based on the regional parameter sets. This was illustrated in Section 2.6.2.8 of companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b) for the area around Wallarah 2 in the Wyong River catchment, where local hydraulic property data from the Wallarah 2 groundwater assessment (Mackie Environmental Research, 2013) were used to constrain model results, and resulted in predictions of drawdown extent, due to the additional coal resource development, at the lower end of the distribution based on the regional parameter sets (see Figure 47 in companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b)). At the 5th percentile, the modelled area of greater than 0.2 m drawdown in the Wyong River catchment is about 3.5 km2 (compared with 56 km2 using regional parameters). Most of this drawdown is associated with the Mandalong Southern Extension project in the adjoining Dora Creek catchment. At the 95th percentile, the area potentially affected by greater than 0.2 m drawdown in the Wyong River catchment is 110 km2, which, while smaller than the 153 km2 estimated using the regional parameters (Table 10), corresponds to a 72% overlap. Drawdowns from the Wallarah 2 and Mandalong Southern Extension projects coalesce when the regional parameters are used, but show a more limited connection when results are constrained using local hydrogeological data. The effect on surface water hydrological response variables, of constraining the regional-scale model results based on local-scale information is picked up again in Section 3.3.3. The implications for impacts on GDEs are considered further in Section 3.4.4.

Table 10 Area (km2) of modelled drawdown in the Wyong River catchment based on regional and locally constrained parameter values